Postfix: Lost connection while receiving initial greeting

First of all, thank you in advance for your help. I had a server running Ubuntu 10.10 with postfix as MTA and mails were being sent out. I just replaced the server wih a new one running Ubuntu 12.10 and suddenly no mails are sent out.
postqueue -p output yields:
(lost connection with mail.example.com[10.XXX.XXX.49] while receiving the initial server greeting)
However when I try telnet 10.XXX.XXX.49 25, I am connected to the mail server and I can send a mail using ehlo, mail from, rcpt to etc.
Any idea what may be wrong?
Thanks.

You put me on the right track. Since our mail server is on a private address, I could not verify whether my IP address was blacklisted with the site you mentioned. So, after investigation with my ISP provider, I found out that they have 2 mail relay servers one on address 10.XXX.XXX.49 and the other on 10.XXX.XXX.50. I figured out that I could not authenticate on the '50' one but it was OK on the '49' one. So, instead of having 'relayhost = Blank' in /etc/main.cf, I added relayhost = 10.XXX.XXX.49 thus forcing mails from my server to relay through address '49'. Now, mails are flowing without problem.